MIT physicists have created a superfluid gas, the so-called Bose-Einstein condensate, for the first time in an extremely high magnetic field. The magnetic field is a synthetic magnetic field, generated using laser beams, and is 100 times stronger than that of the world’s strongest magnets. Within this magnetic field, the researchers could keep a gas superfluid for a tenth of a second — just long enough for the team to observe it.
After cooling the atoms, the researchers used a set of lasers to create a crystalline array of atoms, or optical lattice. The electric field of the laser beams creates what’s known as a periodic potential landscape, similar to an egg carton, which mimics the regular arrangement of particles in real crystalline materials.
When charged particles are exposed to magnetic fields, their trajectories are bent into circular orbits, causing them to loop around and around. The higher the magnetic field, the tighter a particle’s orbit becomes. However, to confine electrons to the microscopic scale of a crystalline material, a magnetic field 100 times stronger than that of the strongest magnets in the world would be required.
The group asked whether this could be done with ultracold atoms in an optical lattice. Since the ultracold atoms are not charged, as electrons are, but are instead neutral particles, their trajectories are normally unaffected by magnetic fields.
Instead, the MIT group came up with a technique to generate a synthetic, ultrahigh magnetic field, using laser beams to push atoms around in tiny orbits, similar to the orbits of electrons under a real magnetic field. In 2013, Ketterle and his colleagues demonstrated the technique, along with other researchers in Germany, which uses a tilt of the optical lattice and two additional laser beams to control the motion of the atoms. On a flat lattice, atoms can easily move around from site to site. However, in a tilted lattice, the atoms would have to work against gravity. In this scenario, atoms could only move with the help of laser beams.
Observation of Bose–Einstein condensation in the Harper–Hofstadter model.
Going forward, the team plans to carry out similar experiments, but to add strong interactions between ultracold atoms, or to incorporate different quantum states, or spins. Ketterle says such experiments would connect the research to important frontiers in material research, including quantum Hall physics and topological insulators.
“We are adding new perspectives to physics,” Ketterle says. “We are touching on the unknown, but also showing physics that in principle is known, but at a new level of clarity.”
Extensions of Berry’s phase and the quantum Hall effect have led to the discovery of new states of matter with topological properties. Traditionally, this has been achieved using magnetic fields or spin–orbit interactions, which couple only to charged particles. For neutral ultracold atoms, synthetic magnetic fields have been created that are strong enough to realize the Harper–Hofstadter model. We report the first observation of Bose–Einstein condensation in this system and study the Harper–Hofstadter Hamiltonian with one-half flux quantum per lattice unit cell. The diffraction pattern of the superfluid state directly shows the momentum distribution of the wavefunction, which is gauge-dependent. It reveals both the reduced symmetry of the vector potential and the twofold degeneracy of the ground state. We explore an adiabatic many-body state preparation protocol via the Mott insulating phase and observe the superfluid ground state in a three-dimensional lattice with strong interactions.
SOURCE – MIT, Nature Physics
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